Photochromic materials are known and exhibit a change in light transmission or color in response to actinic radiation in the spectrum of sunlight. Removal of the incident radiation causes these materials to revert back to their original transmissive state. This phenomenon is known as a photochromic effect. In general, the same photochromic material present in different matrices will produce different colors, different activated intensities and different color activating and fading rates. Methods of using the photochromic effect in the prior art have the following characteristics and drawbacks:
Today, most spectacle lenses are made of a variety of plastics or plastic-glass composites. Most used plastics include PMMA (e.g., Plexiglas by Rohm and Haas, Perspex, Lucite, Altuglas and Optiks by Plaskolite,) and Polycarbonate (e.g., Lexan by General Electric, MERLON by Mobay Chemical Company, MAKROLON by Bayer, and PANLITE from Teijin Chemical Limited).
In one conventional method, a photochromic material is directly mixed with a matrix material, such as an eyeglass lens. Commonly used matrices for the eyeglass lens are allyl diglycol carbonate (CR-39), polycarbonate, and other polymers having high refractive indices. A disadvantage of this method is that it cannot be used where the matrix is not suitable for use as a carrier for the photochromic material. Even if the matrix can be a photochromic dye carrier, it may not be the best carrier to allow particular photochromics to achieve sufficiently fast rates of color change and color fading, a sufficient darkness, or satisfactory color.
To resolve the aforesaid drawback, the use of a coating process to form a photochromic layer on a surface of a lens was developed. Aside from allowing selection of a preferred matrix for the photochromic material, this technique simplifies the manufacturing process and lowers manufacturing costs of the coating layer. In addition to the above description of the photochromic activity, the original color prior to photochromic color change and the resulting color after color change are important factors that determine whether a photochromic article is acceptable in the market. In most cases, a single photochromic dye cannot ensure that after color transformation, its characteristics will be pleasant to the eye.
Some success in rendering plastic ophthalmic lenses photochromic involved embedding a solid layer of photochromic mineral glass within the bulk of an organic lens material. Examples include U.S. Pat. No. 5,232,637 (Dasher, et al.) that teaches a method of producing a glass-plastic laminated ophthalmic lens structure, and U.S. Pat. No. 4,300,821 (Mignen et al.) and U.S. Pat. No. 4,268,134 (Gulati et al.) that teach an ophthalmic lens made of organic material having at least one layer of photochromic mineral glass within its mass to impart photochromic properties to the lens.
Photochromic materials have applications such as sunglasses, graphics, ophthalmic lenses, solar control window films, security and authenticity labels, and many others. The use of photochromic materials, however, has been limited due to (a) degradation of the photochromic property of the materials (fatigue) as a result of continued exposure to UV light, particularly to the shorter and more energetic wavelengths, (b) low photochromic reaction where UV radiation is scarce or when the absorption band of the photochromic dye is narrow and is poorly overlapping with the available spectral part of the solar light. Some partial solution is proposed in U.S. Pat. No. 7,884,992 (Wang et al.), where layered structure of the same matrix and photochromic material using layers of different stoichiometry (different percentage mixtures of the same materials) are used. In this way one can expose to the incoming sunlight the low photochromic material concentration layer and absorb some UV before impinging on the high photochromic material concentration layer.
Photochromic materials are well known and exhibit a change in color, in addition to change in opacity, in response to radiation in the spectrum of sunlight. Many photochromic materials have some initial color or tint in their original transmissive state or in their darker condition. Some of the colors or tints are unpleasant to the human eye.
Common photochromic materials, such as Naphthopyrans and Spirooxazines, are strongly influenced by temperature. Higher temperatures usually lead to faster fade-back times, i.e. the bleaching process after the illumination by activating light has stopped is accelerated with the increase in temperature. Moreover, different dyes have different bleaching rates at room temperature. Essentially, a faster fade-back is usually preferred; however, it is inversely proportional to the amount of tint the material reaches. Since each dye has a different temperature in which its performance is optimized, a combination of different dyes, each in its own layer, may expand the material's working temperature.
Improving the photochromic materials' fatigue resistance, changing their color to be more “eye pleasing,” enhancing their efficiency and expanding their working temperature are important concerns that this invention addresses and provides solutions for.